In computed tomography, X-ray detectors are used for detecting medicinal X-ray radiation; these detectors have a two-dimensional scintillator arrangement or sensor array made of a scintillator material, also known as a fluorescent substance. The scintillator material usually converts the high-energy X-ray radiation into visible light. Suitable fluorescent substances include e.g. rare-earth oxysulfides, which form an oxysulfide ceramic. Moreover, praseodymium, with peaks at 511 nm, 630 nm and 670 nm, is often used for doping the fluorescent substance.
In order to attain a good resolution, the sensor array has a pixel-like structure with a plurality of pixels arranged, in particular, in two dimensions, wherein a pixel generally has a surface of approximately 1 mm×1 mm directed toward an X-ray source. The light produced in the scintillator material of the pixel is subsequently detected and measured by a photosensitive element such as a photodiode, a photomultiplier or a light-sensitive film. The photosensitive elements are likewise arranged in an array, e.g. a photodiode array, corresponding to the pixel-like structure of the sensor array.
Scintillator materials with extremely low afterglow are required for the X-ray detectors used in computed tomography in order to attain a sufficiently high read-out frequency. In order to reduce the so-called afterglow, additionally doping the oxysulfide ceramic with cerium (Ce) has, for example, been disclosed, wherein the Ce content usually lies between 3-50 ppm, preferably between 10-30 ppm. Herein, afterglow is understood to mean the effect that part of the incident X-ray radiation is not converted into light immediately, but with a time delay. As a result of the oxidation of Ce3+ to Ce4+, the ceramic is dyed yellow.
As a result of the yellow coloring of the oxysulfide ceramic, there is a reduction in the emission band at 511 nm, which is to a large extent responsible for an undesired signal drift when detecting the visible light. This signal drift occurs when there is relatively long irradiation and results in, for example, the formation of artifacts in the computed tomography examination recordings. The signal drift is generated by the formation of color centers, which change the absorption around 511 nm. Therefore, signal drift is understood to mean the change of the signal, particularly the reduction of the signal intensity in a certain wavelength range, as a result of an increase in the color centers and thus in the absorption during the course of irradiation. The mechanism of drift reduction by cerium consists of the emission bands at 511 nm being partly absorbed by the yellow coloring of the ceramic and thus the relative change is minimized.
Vacancies in the crystal, where anions are missing, are referred to as color centers. The charge of the missing anions is compensated for by one or more electrons occupying the vacancies. These electrons can absorb electromagnetic radiation in the visible light wavelength range and this is expressed by a discoloring of the crystal—in the case of the oxysulfide ceramic, the latter discolors to yellow. Therefore, a relatively high Ce concentration in the scintillator material both improves the afterglow and reduces the signal drift. However, a disadvantage in this process is that the light yield decreases with increasing Ce concentration.
In a scintillator arrangement of an X-ray detector, reflectors are arranged between the scintillator ceramics of the pixels and these surround the individual pixels on five sides and thus prevent a penetration of the light generated into an adjoining pixel or prevent the light escaping to the outside. The reflectors usually comprise titanium dioxide and epoxy resin as a binding material, wherein titanium dioxide has high reflectivity in the emission region of the scintillator ceramic between 450 nm and 800 nm and so the light quanta or photons generated in the scintillator ceramic are reflected by the reflector when they reach the boundary of the detector element.
As a result of the subdivision of the detector surface into individual pixels surrounded by a light-reflecting material, the path covered by the reflected photons before said photons reach the photosensitive element increases. Therefore, the signal drift in a so-called structured scintillator arrangement having pixels with a size of approximately 1 mm2 increases by a factor of 1.5 to 2.5 compared to its unstructured disk made of a fluorescent material ceramic. This makes the design (sorting) of the X-ray detector relatively complicated because arrays with similar properties have to be grouped next to one another.
DE 44 02 258 A1 describes a fluorescent material, provided for use in computed tomography, based on a rare-earth oxysulfide. In order to reduce the afterglow, the fluorescent material ceramic comprises molybdenum doping with a proportion of approximately 10−1 to 10−6 mole percent in addition to the cerium.